The invention relates to a method of producing flat-blade windshield wipers for motor vehicles with curved flat blades.
Such flat-blade windshield wipers are known from U.S. Pat. No. 3,192,551, wherein one single back of the spring band, being engaged by the wiper arm of the windshield wiper in the center, produces a nearly uniform contact pressure of the rubber wiping strip, secured on its back, on the surface of the windshield over the entire wiping range, such windshield being curved, as a rule. For this purpose, the curved back of the spring band has a material thickness changing over its length, such thickness being the greatest in the center of the back and decreasing toward the two ends of the back, and, furthermore, has a changing back width, which decreases from the center of the back toward the ends thereof.
A method and a device for producing metal strips with varying material thickness are known from WO 94/17932. Such metal strips are used as compoents of windshield wipers. In this process, a strip material having a constant thickness and width is drawn through a pair of counteropposed rolls, whereby the spacing of the rolls is varied in order to shape in this way the strip material with a thickness varying by sections over the length of said material. The material is then finally subjected to a heat treatment.
The method as defined by the invention for producing flat-blade windshield wipers with curved flat blades offers the advantage that flat blades required directly as components of flat-blade windshield wipers, are produced in one single step from a heat-treated spring band blank with varying material thickness.
Flexing and re-flexing takes place in this process as the spring band is continuously advanced, such feed being interrupted briefly periodically for separating the flat blades. The method as defined by the invention supplies final flat-blade products with preset bending radii and largely within exact tolerances, which are absolutely stable dimensionally, and will no longer deform even as they are separated from the band material. Owing to the fact that the center one of the three support sites for the bending of the spring band, and the fourth support site for the re-bending of the spring band each are transversely displaceable relative to the spring band in the direction of the thickness of the band, and that their transverse displacements relative to the spring band are controlled according to preset programs, taking into account the changing material thickness within the flat blades, the spring band undergoes uniform plastic deformation in all regions. It is possible at the same time to realize different bending radii within one flat blade. In the programs for controlling the displacement motion of the two support sites, other parameters influencing the manufacturing result are taken into account as well, in addition to the variable material thickness and, if need be, different bending radii of the flat blades based on zones, such other parameters being, for example the band hardness, the band width, prior alignment of the spring band, sense of winding of the spring band on a supply coil, etc.
For maintaining most exacting manufacturing tolerances, provision is made according to an advantageous further development of the method as defined by the invention that at least some of the separated flat blades are optically measured and compared with nominal value specifications. The mean deviations from the nominal values are used for correcting the bending and re-bending programs.